It has been generally observed that certain gas turbine engines, e.g. aircraft turbofan engines, cool unevenly following shutdown of the engine. Typically, it has been observed that the top of the engine may remain remaining hotter than the bottom of the engine for a period of time after shutdown. Because of convection effects the bottom part of the engine cools more quickly than does the top part. It has been further observed that this uneven cooling may cause a top of a shaft in the turbine being hotter than the bottom of the shaft.
Owing to thermal expansion of the material, the net result of this temperature differential can often be that the shaft becomes distorted, where typically this distortion is in the form of a “bowing” (sometimes referred to as “hogging”), usually upwardly, of the shaft. The shaft may become eccentric relative to its axis, taking on an effectively arcuate shape instead of being generally cylindrical. This distortion may become problematic if a start is attempted with an aviation gas turbine engine in this condition. In such instances, the shaft eccentricity will result in vibrations which may be felt in the airplane, or may result in rubbing of a rotor blade or fan blade so as to negatively affect performance of the turbine.
Methods proposed for dealing with the bowing problem have included provision of a drive to keep the engine rotors turning for some time after the engine has shut down. This may, in itself, require the expenditure of a considerable amount of power and has other potentially undesirable effects. One specific method for addressing the problems attendant to starting a gas turbine engine having a shaft in a distorted or bowed state, is to “motor” the affected shaft, such that it is powered and rotated to a running speed (usually a continuous running speed) during the start procedure and prior to addition of fuel to the turbine. Such motoring has been beneficial in bringing in airflow into the engine core, so that any temperature disparity can be evened out. However, such extended motoring may be undesirable owing to the consumption of power. Furthermore, such extended motoring typically lengthens the start time, and undesirable bowed-rotor vibration may be encountered during the motoring. This may be especially problematic in some newer turbofan engines.
Thus, there remains a need to shorten the time needed to start a gas turbine engine in such a way that bowing of a shaft is not a significant problem.